The invention relates to a stepping motor of a wristwatch which motor is mounted on a support plate and which, with regard to its magnetic circuit, consists of a magnetic coil, a magnetic core disposed in the coil, an armature for conducting the magnetic flux from the magnetic core and a rotor enclosed by the armature, the armature being divided by a gap extending along a plane receiving the rotor's axis.
Such stepping motor arrangements are generally rectangular in shape wherein the coil is arranged along one of the rectangle's longer sides while the rotor is arranged in the other longer side of the rectangle. In this arrangement the side provided with the coil is the magnetic core from which extend L-shaped armature portions which have short legs disposed adjacent the ends of the coil core or in engagement with the ends of the coil core and which have pole pieces for the rotor formed at the ends of the other legs.
Manufacture of the coil with the coil core is possible in two ways: Firstly, the coil may be directly wound onto the coil core or, secondly, a hollow coil may be made into which the core is then inserted. Although manufacture of such hollow coils is possible in an automated process with hardly any difficulties, it is advantageous to wind the coil directly onto the core, especially for use in wristwatches since this provides for better space utilization. Such direct winding of the coil onto the core however is only possible if the radial area around the coil is free of obstructive components as is the case for example in the arrangement described in DE AS No. 2434452 which has coil core end portions that are bent over like horns such that the area between the bentover portions is free of obstructive components. After the winding of the coil the armature portions are placed onto those bent-over portions and screwed thereto. Such an arrangement however has the disadvantage of having two connecting areas so that there are two points of magnetic resistance between the coil core and the armature members. The arrangement also requires difficult and timeconsuming positioning of the parts.
Generally it is noted that proper positioning of the pole pieces at the ends of the armature members relative to the rotor is of utmost importance for proper operation of the motor. If the pole pieces are not properly positioned relative to the rotor, there are not only changes in the magnetic line circuit which might cause immediate malfunction of the stepping motor but which may result in later mechanical problems especially with the presence of some impurities. In order to prevent such problems, armatures have been proposed which do not consist of two parts but which are unitary and provided with an opening for the reception of the rotor. In order to provide for the necessary interruption of the magnetic circuit path at the rotor location, the armature structure is very thin, that is, it provides only for narrow interconnecting sections around the rotor so as to generate a relatively high magnetic resistance in this area. Nevertheless a relatively large amount of magnetic field lines may still pass through the interconnecting sections, though narrow, and these magnetic lines are lost for the generation of rotor drive forces. As a result the energy consumption of the motor is relatively high, that is, the life span of the battery included in the wristwatch is relatively short. It is noted however that assembly of the watch is facilitated and is easily possible with automatic manufacturing equipment since, after sliding the coil onto the core, only connecting members need to be placed between the armature and the coil core. This arrangement however still has a number of connecting points which require accurate positioning.
It is the object of the present invention to provide a watch stepping motor with a divided armature which will permit the winding of the coil or the sliding of a pre-wound coil onto the core and which does not require difficult position adjustments so that the stepping motor may be assembled by robots.